In recent years, the universal serial bus (USB) has become one of the most widely used techniques for interconnecting electronic devices. Originally used to interconnect computers and standard peripheral devices (e.g., printers, disk drives, etc.), USB has grown to support a vast array of portable USB devices (e.g., cellular telephones, personal digital assistants, cameras, personal music players, etc.).
USB devices are connected to a USB host (e.g., a PC, laptop, tablet PC, etc.) via USB ports. Generally, one USB host can support up to seven USB devices. In cases where more than seven USB devices are required, a USB hub may be used. Each USB hub may be connected to its own set of seven USB devices (or hubs as necessary). The USB connections between the USB host, USB hubs and USB devices allow data, e.g., names, phone numbers, calendars, photographs, music, etc., to flow between the USB host and USB devices.
Generally, a USB connection has a DC power line commonly referred to as the VBUS, a ground, and a twisted pair of data lines commonly referred to as D+ and D−. According to the USB 2.0 standard, a valid USB host should supply 5 volts and no more than 500 milliamps of current. Further, USB ports are supposed to provide current-limit protection, which can be set as high as 5.0 amps and as low as 100 milliamps. However, not all USB ports adhere to the USB standard. Further, USB hosts have different types of ports, including AC-powered ports (e.g., in host PCs and powered hubs), non-powered ports (e.g., in notebook computers operating on battery power), passive ports (e.g., in passive hubs), etc. Each port type has a different power delivery capability. For example, an AC-powered port may source 5 volts and 500 milliamps of current or more. A non-powered (battery-powered) port may source 5 volts, but may share 500 milliamps of current with other USB ports on the USB host. Thus, a non-powered port often sources less than 500 milliamps of current. A passive port may source 5 volts and only 100 milliamps of current.
Typical USB devices have a rechargeable lithium-ion/polymer battery for sourcing operating power and a control system to re-charge the battery. According to the USB standard, a USB device may take power from the power line of the USB port both to operate its system core and to re-charge its battery. A USB device with a battery charger typically includes an apparatus for regulating charge current to prevent the charging process from drawing excessive current and starving the remainder of the USB device. Charge-current regulation is typically accomplished using two current sense resistors to monitor the charge current to the battery and the load current to the system core. While effective, this mechanism has drawbacks. Among these is the fact that the battery charger can select only from one of two charging currents, namely, 500 milliamps or 100 milliamps. At 100 milliamps, battery charging could take as long as 8-10 hours for an 800-milliamp battery. Users may become irritated and return USB devices and/or USB chargers as defective. Also, when a USB host or hub is capable of supplying more than 100 milliamps but less than 500 milliamps, traditional charge-current regulation operates as though extra capability did not exist. Thus, traditional charge-current regulation unnecessarily slows the charging process.
A system and method are needed to facilitate more effective use of USB potential for charging a battery of a battery-powered USB device.
Similar problems exist for typical AC adaptor power supplies. An electronic device that uses a rechargeable battery is typically supplied with an AC power adaptor rated to support the charge-current demands of the electronic device. However, some electronic devices are not supplied with a power adaptor and, in many cases, third party power adapters are used. Using a non-approved power adapter on an electronic device risks starving the system core of the device due the drawing of excessive charge-current. Thus, when working with battery chargers, USB port and AC power adapter capabilities must assure that the battery charging process does not starve system operation.